The proposed research will explore a new approach to diminish breast cancer metastasis by targeting G protein-coupled receptors (GPCRs) that control the prenylation of the small GTPase Rap1B. Prenylated Rap1B localizes at the plasma membrane of breast cancer cells and stabilizes adherens junctions, which promotes cell-cell adhesion and diminishes the release of metastatic cells from breast tumors. We recently discovered that Rap1B prenylation is suppressed by activating adenosine A2B receptors (A2BR) on breast cancer cells. We reported that A2BR activation causes protein kinase A to phosphorylate newly synthesized Rap1B, which decreases the interaction of Rap1B with the chaperone protein SmgGDS and prohibits Rap1B from entering the prenylation pathway. These events promote the cytosolic and nuclear accumulation of non-prenylated Rap1B, resulting in loss of cell-cell adhesion and acquisition of a metastatic phenotype. This GPCR-mediated pathway that promotes the metastatic phenotype is very novel, because it was previously thought that cells do not possess signaling mechanisms to suppress prenylation, and it was thought that suppressing the prenylation of small GTPases would inhibit, rather than promote, metastasis. Based on our discovery, inactivating A2BR should restore Rap1B prenylation and trafficking to the cell membrane, which will increase cell- cell adhesion and diminish the release of metastatic cells from the tumor. We recently discovered that other GPCRs (in addition to A2BR) can suppress Rap1B prenylation in breast cancer cells. These GPCRs are inactivated by drugs that are currently being given to patients with diseases other than cancer. We hypothesize that these drugs can be used to inactivate the GPCRs that suppress the prenylation of Rap1B in breast cancer cells, resulting in increased Rap1B prenylation, increased cell-cell adhesion, and diminished metastasis. This hypothesis will be tested by 1) identifying GPCRs that promote the metastatic phenotype of cultured breast cancer cells by suppressing the prenylation and membrane trafficking of Rap1B, 2) testing drugs that inactivate these GPCRs, to determine if they will diminish breast cancer metastasis in animal models by restoring normal Rap1B prenylation in the tumor cells, and 3) examining patients' breast tumors to determine if expression of specific GPCRs and diminished Rap1B prenylation are associated with specific tumor subtypes and a more aggressive stage. Accomplishing these goals will provide significant benefits by developing an innovative strategy to inhibit breast cancer metastasis.